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Re: USB pass through into Mac OS 9.x with qemu-system-ppc

From: Gerd Hoffmann
Subject: Re: USB pass through into Mac OS 9.x with qemu-system-ppc
Date: Wed, 17 Feb 2021 08:27:16 +0100

> Hi Gerd,
> Thanks for looking into this. It looks to me that the usb storage
> device nicely reports endpoints 1 and 2 when asked, but that the host
> only ever communicates with endpoint 1.

EP 1 is host -> device.
EP 2 is device -> host.

So the host sends requests but the device never answers, probably due to
the wMaxPacketSize mismatch, macos being confused and sending broken
requests ...

> Can this also explain that other (non-mass-storage) devices cannot be
> passed through successfully ?

Yes, you can have simliar issues with other devices.  Devices can have
different interface descriptors for different device speeds.  This can
be rather small differences like the wMacPacketSize for usb sticks.  The
descriptors can be identical.  Or there can be large differences, like
usb-audio devices offering more channels when plugged into a usb2 port
(with enough bandwidth for that).

Problem is when the device is plugged into a usb2 port you can't query
the usb1 descriptors.  So qemu presents the wrong descriptors to the
guest in case host and guest use different usb speeds.  That may or may
not work ...

The other way around is less problematic, when plugging a usb2 device
into a usb3-capable (xhci) port I can tell the guest "this is a usb2
device".  But reporting "this is a usb2 device" via ohci isn't going to
fly for obvious reasons ...

take care,

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